A linear drive is known from DE 44 15 103 A1, corresponding to U.S. Pat. No. 5,641,131. The linear drive has a cylinder tube with an end wall provided with an opening. A piston is arranged shiftably in the cylinder tube. The piston comprises a hollow inner space. A pyrotechnical propellant charge is provided which following its ignition acts upon the inner space of the piston with compressed gas. When required, the propellant charge is ignited so that the piston as affected by the generated gas executes a tensioning stroke which is translated from the belt tensioner into a rotation of a belt reel of a belt retractor in the coiling direction, as a result of which the so-called belt slack is taken up from the belt system so that in case of a collision the vehicle occupant is involved in the vehicle delay as early as possible.
One problem with such a linear drive involves assuring that even when the propellant charge is ignited under unfavorable conditions, e.g. misuse, overheating due to a vehicle fire or blockage of the spindle of the connected belt retractor, the propellant charge of parts of the linear drive pose no risk of injury whatsoever. It is known from DE-GM 295 20 307 to provide the face wall of the piston with a passage which is covered by a bursting disc on the side of the inner space of the piston. When an excessively high pressure exists in the inner space of the piston the bursting disc opens up the passage so that this pressure is able to escape from the inner space of the piston and prevent bursting of the cylinder tube without the latter needing to be designed particularly rugged. In this arrangement, dimensioning the bursting disc accordingly enables the pressure occurring in the inner space of the piston to be set as a maximum and adapted to the conditions in each case. The bursting disc is furthermore provided with a calibrated gas exit opening opposite the passage. In addition to restricting the pressure occurring maximally in the inner space of the piston this design also offers advantages as regards the response of the linear drive after a belt tensioning action has occurred when the belt reel is connected to a means of energy absorption such as a torsion bar which absorbs load peaks in the belt system.
The problem remains, however, that following ignition of the propellant charge a flow of hot gas emerges from the opening in the bursting disc which involve a flame. When the bursting disc is broken open due to a prohibitively high pressure, fragments may be catapulted out of the cylinder tube. By the present invention these problems too, are eliminated.